(1) Field of the Invention:
This invention relates to metal vapor discharge lamps useful in photochemical reactions or for hardening paints and inks.
(2) Description of the Prior Art:
Ultraviolet rays are often used to induce photochemical reactions or to harden paints and inks. For these purposes, ultraviolet rays having a wavelength range of about 280-400 nm are effective.
As sources for such ultraviolet rays, electrode-type high-pressure mercury vapor discharge lamps in each of which a discharge takes place between its electrodes have conventionally been used. Furthermore, electrodeless high-pressure mercury vapor lamps have also been known recently, in each of which mercury and a rare gas are filled in amounts sufficient to retain a discharge within an electrodeless light-emitting tubing, and electromagnetic waves such as high-frequency waves of several tens MHz or higher or microwaves are externally irradiated to the tubing so as to excite the internally-filled mixture and to produce a discharge as light.
By the way, each of these electrode-type or electrodeless high-pressure mercury vapor lamps emits light having a line spectrum which comprises a number of peaks and extends over a considerably wide wavelength range. It is therefore inefficient to use high-pressure mercury vapor lamps for photochemical reactions or hardening of paints, which require the above-mentioned range as an effective wavelength range. Accordingly, there are often employed metal vapor discharge lamps in each of which a metal halide is filled as a light-emitting material within the light-emitting tubing so as to increase the light quantity of a specific effective wavelength range. Metal vapor discharge lamps in which iron is filled are particularly convenient for such purposes as photochemical reactions and hardening of paints, because the lights from these lamps is continuously emitted in the range of the 350-400 nm.
When a metal vapor discharge lamp filled with iron in an amount of enough to provide an iron-related spectrum of sufficient intensity is lit over many hours, iron is caused to deposit on the inner wall of its light-emitting tubing so that a thin film is formed thereon. For this reason, the amount of iron which contributes to the emission of light decreases and at the same time, the thus-formed thin film prevents the transmission of ultraviolet rays, leading to a problem that the output of ultraviolet rays is reduced.
With a view toward improving this problem, it has been proposed to additionally incorporate and fill lead within a metal vapor discharge lamp which contains iron therein (Japanese Utility Model Publication No. 15503/1979). Although such an addition of lead can prevent the formation of a thin iron film and can retain the output of ultraviolet rays as intended, the output of large bright lines of 302 nm, 313 nm and 365 nm from mercury is extremely weakened by the thus-added lead. It is hence not desirable to add lead. Such lead-added metal vapor discharge lamps are not suited for such purposes as photochemical reactions or hardening of paints.
It has also been known to add tin. Similar to lead, tin is accompanied by a drawback that the output of the line spectrum of mercury is lowered to a significant extent.